Condensation of secondary organic compounds onto ultrafine aerosols is
important for growing these particles to sizes where they can act as cloud
condensation nuclei. The organic flux to ultrafine particles depends strongly
on the volatility of the condensing compounds. This paper presents
quantitative estimates of the volatility of secondary organic aerosol (SOA)
in freshly nucleated particles. We examine 13 nucleation/growth events in two
remote continental locations, Hyytiälä, Finland and Egbert, ON,
Canada. Two independent methods are used to quantify the volatility of the
growing nucleation mode: (1) modelling of the growing nucleation mode to
determine which volatilities allow the model to reproduce observed growth,
and (2) modelling of the evaporation of heated aerosols in a Volatility
Differential Mobility Particle Sizer to determine which volatilities allow
the model to reproduce the observed evaporation. We find that the average
saturation vapor concentration (<i>C</i>*) in the freshly nucleated
particles (once <i>D</i><sub>p</sub> > 3 nm) is likely less than
10<sup>−3</sup>&ndash;10<sup>−2</sup> μg m<sup>−3</sup> (this corresponds to 3 × 10<sup>6</sup>&minus;3 × 10<sup>7</sup> molecules cm<sup>−3</sup> and a saturation vapor
pressure of 10<sup>−8</sup>&ndash;10<sup>−7</sup> Pa). This maximum volatility depends somewhat
on other uncertain factors that affect the size-dependent condensation of
secondary organic compounds such as the surface tension, mass accommodation
coefficient and the volatility of the pre-existing aerosols. However, our
tests suggest that under no reasonable assumptions can the SOA in the
ultrafine particles contain a majority of compounds with <i>C</i>* > 10<sup>−2</sup> μg m<sup>−3</sup>. We demonstrate that the growth could be driven by
either gas-phase or particle-phase chemistry but cannot conclude which is
responsible for the low-volatility SOA.